Russia on Friday seized a consignment of the radioactive isotope Sodium-22 used for medical and research purposes at a Moscow airport from a Tehran-bound passenger, the customs service said.
"Tests showed that the Sodium-22 could only have been obtained as the result of the work of a nuclear reactor," it said in a statement. "A criminal enquiry has been opened and the materials transferred to prosecutors."
Customs had been alerted by a warning system at Sheremetyevo airport ahead of the Moscow to Tehran flight that background radiation in the departures hall was 20 times the norm. A passenger's bag was then searched.
"Eighteen metallic objects of industrial origin were found, packed into individual steel boxes," it said.
"Tests then found that the objects were in fact the radioactive isotope Sodium-22 that had been machine-produced."
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No further details were immediately available on the consignment or the identity of the passenger who was carrying the materials.
Spokesman for Russian state nuclear agency Rosatom Sergei Novikov said Sodium-22 was used for medical and research purposes and produced in so-called cyclotrons, a type of particle accelerator, and not in nuclear reactors.
"There are a lot of cyclotrons in Russia," he told AFP, adding that they were used in medical and educational establishments where security levels were not as tight as those at nuclear reactors.
Rosatom only supplies Iran with medical isotopes, molybdenum-99 and iodine-131, Novikov added, without providing further details.
Russia has relatively close ties with Iran and has built its first nuclear power station in the southern city of Bushehr. Moscow has also delivered the nuclear fuel for the reactor.
Moscow has echoed Western concerns about the nature of the Iranian nuclear programme but has stopped short of publicly accusing Tehran of seeking atomic weapons and always said that the standoff should be solved by diplomacy.
Experts have long called for tight controls against nuclear smuggling so Iran cannot get hold of materials it is barred from obtaining under UN Security Council sanctions.